Semiconductor integrated circuit device fabrication normally requires repeated processes of photolithographic masking and etching on an upper surface of a wafer of a semiconductor material such as silicon. Each of these processes requires spreading a thin layer of a liquid photoresist material on the surface of the wafer, selectively exposing it through a mask to actinic light, developing the photoresist to define a mask pattern on the wafer surface and using this mask pattern to describe a permanent pattern on the wafer. The permanent pattern on the wafer may in turn be used to control selective etching, diffusion, ion implantation, metal deposition, oxide deposition, and other processes that describe the finished integrated circuit.
As integrated circuits have become more complex, the requirements for accuracy and resolution of the photolithographic processing have become more stringent. Requirements for line-width definition of less than one micron are becoming increasingly commonplace. As a consequence, greater accuracy is required at all stages of the photolithographic processing.
The liquid photoresist is typically dispensed on one surface of the wafer with the wafer being held in a vacuum chuck that is subsequently rotated to disperse the liquid over the entire surface. While this method is suitable for many purposes, it does tend to result in non-uniformities in thickness of the photoresist that can affect the uniformity and consequent accuracy of the resulting pattern to be developed. Development of the photoresist likewise requires a uniform distribution of developing fluid for a uniform development of the desired pattern. Finally, the steps of stripping photoresist, liquid etching, and cleaning of the wafer all require a uniform distribution of fluid. The effects of non-uniform distribution of any of these liquids can be somewhat overcome by dispensing rather large amounts of the liquid at different locations over the surface, but in the mass-production of integrated circuits, this solution can be quite wasteful of the liquid and therefore expensive, and in any event, it only reduces rather eliminates coating thickness non-uniformities.